Styrene-butadiene rubber (SBR) is widely utilized in manufacturing tires for automobiles, trucks, aircraft and other equipment. SBR can be synthesized by utilizing emulsion polymerization techniques. Typical emulsion systems employed in the synthesis of SBR contain water, an emulsifier (soap), a free radical generator, styrene monomer, and 1,3-butadiene monomer. For example, in free radical emulsion polymerization systems radicals can be generated by the decomposition of peroxides or peroxydisulfides.
Commonly employed initiators include t-butyl hydroperoxide, pinane hydroperoxide, para-menthane hydroperoxide, potassium peroxydisulfate (K.sub.2 S.sub.2 O.sub.8), benzoyl peroxide, cumene hydroperoxide and azobisisobutyronitrile (AIBN). These compounds are thermally unstable and decompose at a moderate rate to release free radicals. The combination of potassium peroxydisulfate with a mercaptan such as dodecyl mercaptan is commonly used to polymerize butadiene and SBR. In hot recipes, the mercaptan has the dual function of furnishing free radicals through reaction with the peroxydisulfate and also of limiting the molecular weight of polymer by reacting with one growing chain to terminate it and to initiate growth of another chain. This use of mercaptan as a chain transfer agent or modifier is of great commercial importance in the manufacture of SBR in emulsion since it allows control of the toughness of the rubber which otherwise may limit processibility in the factory.
A standard polymerization recipe agreed on for industrial use became known as the "mutual," "standard," "GR-S" or "hot" recipe and was as follows (based upon parts by weight):
______________________________________ Butadiene 75.0 Styrene 25.0 n-Dodecyl Mercaptan 0.5 Potassium Peroxydisulfate 0.3 Soap Flakes 5.0 Water 180.00 ______________________________________
When this standard recipe is employed in conjunction with a polymerization temperature of 50.degree. C., the rate of conversion to polymer occurs at 5-6 percent per hour. Polymerization is terminated at 70--70 percent conversion since high conversions led to polymers with inferior physical properties, presumably because of crosslinking in the latex particle to form microgel or highly branched structures. This termination is effected by the addition of a "shortstop" such as hydroquinone (about 0.1 part by weight) which reacts rapidly with radicals and oxidizing agents. Thus the shortstop destroys any remaining initiator and also reacts with polymer free radicals to prevent formation of new chains. The unreacted monomers are then removed, first the butadiene by flash distillation at atmospheric pressure, followed by reduced pressure and then the styrene by steam stripping in a column. A dispersion of antioxidant, such as N-phenyl-.beta.-naphthylamine (PBNA) is typically added (1.25 parts) to protect the SBR from oxidation. The latex can then be partially coagulated (creamed) by the addition of brine and then fully coagulated with dilute sulfuric acid or aluminum sulfate. The coagulated crumb is then washed, dried and baled for shipment. One of the first major improvements on the basic process was the adoption of continuous processing. In such a continuous process, the styrene, butadiene, soap, initiator and activator (an auxiliary initiating agent) are pumped continuously from storage tanks into and through a series of agitated reactors maintained at the proper temperature at a rate such that the desired degree of conversion is reached at the exit of the last reactor. Shortstop is then added, the latex is warmed by the addition of steam and the unreacted butadiene is flashed off. Excess styrene is then team-stripped off and the latex is finished, often by blending with oil, creaming, coagulating, drying and bailing.
For further details on SBR and the "standard recipe," see The Vanderbilt Rubber Handbook, George G Winspear (Editor), R T Vanderbilt Company, Inc (1968) at pages 34-57.